1. Field of the Invention
The present invention relates to glazing in the broad sense of the term (any substantially transparent substrate or assembly of substrates) which is provided with thin layers having thermochromic properties.
The invention relates more particularly to thermochromic layers based on vanadium oxide.
2. Description of the Background
Vanadium oxide has already been much studied. It is a material which, as the temperature rises, switches at a temperature of about 70° C. from electrically insulating behaviour to electrically conducting behaviour due to a crystallographic distortion. This change in its electrical properties is accompanied by a change in its optical properties, essentially in the infrared (when the temperature of the layer exceeds the switch temperature, the layer becomes reflecting and absorbent in the infrared). This is one of the rare thermochromic compounds known to have such a transition at a temperature relatively close to room temperature, hence the interest that it has aroused in the production of glazing which can, for example, filter out solar radiation in hot weather.
To lower the switch temperature of vanadium oxide to temperatures of about 25 to 55° C., a more appropriate temperature range for such a role, it has been proposed, for example in U.S. Pat. No. 4,401,690, to incorporate into it a dopant such as niobium, tantalum, molybdenum, iridium or tungsten. It has also been proposed, in Patent Application WO99/62836, to incorporate both tungsten and fluorine into the vanadium oxide.
However, when this material is used by itself, in a thin layer, there are not many possible ways to modulate jointly its optical and thermal, or even electrical, properties, depending on whether the temperature is above or below its switch temperature.
It is therefore an object of the invention to provide greater flexibility in the various properties of this thermochromic material, especially so as to be able to give glazing which has been provided therewith more modifiable functionalities or even novel functionalities.